The present invention describes a process for producing hard, radiation-curable coating systems based on aqueous polyurethane dispersions, the coating systems obtainable by the process, the use of the coating systems as coating materials and/or adhesives, and articles and substrates which comprise these coating materials and/or adhesives.
Radiation-curable, aqueous coating systems based on a polyurethane polymer find application in the coating of substrates including wood, plastics and leather, and are distinguished by a multiplicity of positive qualities, such as high chemical resistance and mechanical stability. A particular advantage is the ultra-rapid curing, within seconds, of the polyurethane topcoat by crosslinking of the ethylenic double bonds present in the polymer, by means of high-energy radiation.
If aqueous, radiation-curable polyurethane dispersions are used in coating systems for pigmented coating materials, the photochemical curing is hindered as a result of the absorption and scattering of the pigment. As a result, the coating only partially cures, and the high quality properties of a transparent polyurethane coating material are not achieved.
EP-B 753 531 and EP-B 942 022 describe polyester acrylate urethane dispersions based on hydroxyl-containing polyester acrylates, polyether acrylates and polyepoxy acrylates. Oligomethacrylates, of the kind set out in this specification as component (E), are not used in EP-B 753 531 and EP-B 942 022. The coating systems described are suitable for physically drying transparent coating materials, more particularly for wood and plastic applications. Where binder systems of this kind are pigmented, inadequate reactivity results in low hardness and inadequate resistance towards chemicals and coloring liquids.
Aqueous, radiation-curable polyurethane dispersions based on hydroxyl-containing polyester acrylates and polyether acrylates are found in EP-B 872 502. There again, the preferred application of the coating systems is in transparent coating materials. Oligomethacrylates of the kind set out in this specification as component (E) are not used in EP-B 872 502. The pigmented coating materials of Example 10 evidence sensitivity to coffee, mustard and red wine at a pigment content of just 10% by weight. The sensitivity to marks is said to be improvable through application of an additional transparent coating material. The hardness of the pigmented system is not addressed. In contrast to all of the other examples, elasticity and hardness are not addressed.
EP-B 1 311 639 describes the production and use of a coating system for veneer that is based on polyurethane dispersions. Besides a polyurethane acrylate, the dispersions comprise a solvent component, preferably N-methylpyrrolidone. A description is given of their application in the postforming process, where an important factor is high flexibility on the part of the veneers. Even as a transparent coating material, the coverings obtained are very soft. Pigmented systems based on the cited binders are significantly less suitable. Furthermore, on account of its teratogenic effect, the use of N-methylpyrrolidone is undesirable. Any indication that, without the use of such a solvent component, aqueous polyurethane dispersions are obtained that are suitable for the use of hard coating systems is absent from EP-B 1 311 639.
EP-B 181 486 describes polyurethane dispersions which are based on polyether-, polyester-, polylactone- and carbonate diols and/or mixtures thereof but carry no acrylate functions on the polyurethane backbone. By admixing acrylate-containing compounds, an aqueous, UV-curable polyurethane dispersion is obtained for the high-gloss coating of leather. Since it is not possible for photo-induced crosslinking to take place between the acrylate-containing compounds and the polyurethane, the resulting applied films are very soft and flexible and of low chemical resistance. Any disclosure to the effect that aqueous polyurethane dispersions which carry acrylate functions on the polyurethane backbone can be used for producing hard coating systems is absent from EP-B 181 486.
For everyday products that are confronted by severe challenges, such as mobile phone casings, for which minimal scratchability, maximum chemical resistance, adhesion and appealing optical qualities are required, the coating-systems employed are still always solvent-borne coating systems, since aqueous binders, such as in the systems cited here, do not yet achieve sufficiently good outcomes in the entirety of their properties, such as high gloss and sun cream resistance. Here as well a switch to aqueous systems is called for on environmental grounds.
The object of the present invention is therefore to provide a process that makes it possible to provide coating systems which are based on an aqueous, radiation-curable dispersion, are pigmentable and are distinguished by mechanical stability, chemical resistance and high gloss.